Image forming apparatus

ABSTRACT

An image forming apparatus that includes a plurality of image bearing members, a plurality of drive sources each including a drive gear, a plurality of driven gears, a visible image forming unit, an endless traveling member, a transfer unit, an image detection unit, and a controller. The controller controls rotation of the plurality of image bearing members according to a velocity fluctuation pattern of each surface of the image bearing members based on a detection time interval between predetermined visible detection images formed on the surface of the image bearing member and transferred therefrom to the endless traveling member detected by the image detection unit. Each of the plurality of driven gears includes a gear portion and an engaging portion integrated therewith. The gear portion includes a geared circumference and the engaging portion engages the image bearing member.

CROSS-REFERENCE TO RELATED APPLICATION

This patent specification is based on and claims priority from JapanesePatent Application No. 2007-004090 filed on Jan. 12, 2007 in the JapanPatent Office, the entire contents of which are hereby incorporated byreference herein.

BACKGROUND

1. Field of the Invention

The present invention relates to an image forming apparatus.

2. Description of the Related Art

In an image forming apparatus such as a copier, a facsimile machine, ora printer, visible images are formed, for example, on each of aplurality of rotating image bearing members such as photosensitiveelements and transferred at transfer positions to an endless travelingmember such as an intermediate transfer belt or a recording medium heldon the endless traveling member, such that the visible images aresuperimposed one atop another. In this type of the image formingapparatus, each of the visible images may be displaced from each otherin a sub-scanning direction, i.e., the direction of rotation of theimage bearing member, during transfer due to, for example, aneccentricity of a driven gear that rotates coaxially with the imagebearing member and transmits a rotary drive force to the image bearingmember. Specifically, a driven gear that has an eccentricity causesfluctuation in the velocity of the image bearing member. The velocityvaries in sine wave form with a cycle of a rotation lap of the imagebearing member. This is because, when a driven gear having aneccentricity is meshed with a drive gear of a drive motor, the linearvelocity of the surface of an image bearing member engaging the drivengear is slowest at the point where the radius of the driven gear isgreatest, and fastest at the point where the radius of the driven gearis shortest, and both points are 180 degrees apart from each other withrespect to the rotation shaft of the driven gear and the image bearingmember.

A dot formed on an image bearing member that rotates at a fastervelocity arrives at the transfer position earlier than usual. Bycontrast, a dot formed on an image bearing member that rotates at aslower velocity arrives at the transfer position later than usual.Accordingly, for example, a transferred sooner-than-usual dot isoverlapped onto a transferred later-than-usual dot from a differentimage bearing member, or a transferred later-than-usual dot isoverlapped onto a transferred sooner-than-usual dot. This causes dotdisplacement, resulting in image displacement in the sub-scanningdirection.

There is known an image forming apparatus that can rotate an imagebearing member based on a drive velocity pattern that cancels thevelocity fluctuation pattern thereof that causes such imagedisplacement. The mechanism involves: Forming detection toner imagesarranged on the surface of a drum-like image bearing member with aparticular interval in the surface moving direction thereof;transferring the images to a transfer belt; detecting each detectiontoner image on the transfer belt by a photosensor; detecting thevelocity fluctuation pattern per rotation lap of the image bearingmember based on the detected intervals between the detection tonerimages; determining the drive velocity pattern that cancels the velocityfluctuation of the image bearing member; and driving the image bearingmember based on the drive velocity pattern when an image is formed usingimage information sent from a personal computer, etc. When an imageforming apparatus has multiple image bearing members, the drive velocitypattern is determined for each of the image bearing members.

There is known another image forming apparatus that can prevent imagedisplacement caused by velocity fluctuation of photosensitive elementsby relatively synchronizing phases of the velocity fluctuation patternsthereof. Similar to the above-described image forming apparatus, thevelocity fluctuation pattern per rotation lap of the photosensitiveelement is detected based on detected intervals between detection tonerimages. At the same time, a reference mark provided to the driven gearthat rotates coaxially with the photosensitive element is detected byanother photosensor to detect when rotation of the photosensitiveelement arrives at a particular angle. The relation between suchdetected rotation timing and the phase of the velocity fluctuationpattern is determined for each photosensitive element, on the basis ofwhich the phase difference between the velocity fluctuation patterns ofthe photosensitive elements is adjusted by temporarily changing thedriving velocity of drive motors that drive the respectivephotosensitive elements. By this temporary change, images arriving atthe transfer positions sooner than usual, or images arriving at thetransfer positions later than usual, can be synchronized with eachother. Thus, image displacement can be prevented.

When photosensitive elements are arranged in an image forming apparatusat an interval that is an integral multiple of the circumference of thephotosensitive element, each photosensitive element rotates integraltimes while a toner image on, for example, a recording medium is movedfrom one transfer position to the transfer position of the next tonerimage. Therefore, by adjusting the phase difference between the velocityfluctuation patterns of the photosensitive elements to zero, the imagesare appropriately overlapped at each transfer position. When thephotosensitive elements are not arranged at an interval that is anintegral multiple of the circumference of the photosensitive element,dots are appropriately overlapped at each transfer position by providinga phase difference with a particular period of time to the velocityfluctuation pattern of each photosensitive element.

However, there are some cases in which driving each photosensitiveelement according to the drive velocity pattern or adjusting the phaseof the velocity fluctuation pattern of each photosensitive element isnot sufficient to prevent image displacement. The reason for this is asfollows.

A typical photosensitive element is structured to be easily attached toand detached from an image forming apparatus to improve maintenanceefficiency. By comparison, a driven gear that rotates coaxially with thephotosensitive element and transmits a rotary drive force to thephotosensitive element is rotatably fixed to the image formingapparatus. When the photosensitive element is installed in the imageforming apparatus, one end of the rotation shaft of the photosensitiveelement engages the driven gear. The driven gear in the image formingapparatus having the above-described configuration includes a tubularengagement portion and a disk-like gear portion. The engagement portionis fitted into and protrudes from the center of the gear portion alongthe axial direction.

FIG. 1 is a perspective view illustrating a photosensitive element 3 anda photosensitive element gear 133 included in a typical image formingapparatus. The photosensitive element 3 is included in a process unit,not shown, that is detachably installed in the image forming apparatus.The rotation shaft of the photosensitive element 3 protrudes from bothsides in the axial direction of the drum portion of the photosensitiveelement 3. At one end of the rotation shaft, a coupling 3 b is formed toengage an engaging portion 133 b included in the photosensitive elementgear 133.

The photosensitive element gear 133 is rotatably fixed to the imageforming apparatus and includes a disk-like gear portion 133 a having ageared circumference, not shown, and the engaging portion 133 b thatengages the coupling 3 b of the photosensitive element 3. The engagingportion 133 b has a size in the rotating axial direction to engage thecoupling 3 b, which slides in the rotating axial direction when theprocess unit is assembled. Therefore, the photosensitive element gear133 is configured such that the engaging portion 133 b significantlyprotrudes in the axial direction from the center of the disk-like gearportion 133 a.

FIG. 2 illustrates the photosensitive element gear 133 included in atypical image forming apparatus. In FIG. 2, an insertion hole 133 c thatreceives the base side of the engaging portion 133 b is formed at thecenter of the disk-like gear portion 133 a. Around the insertion hole133 c, a pin groove 133 d is formed to receive a pin.

The engaging portion 133 b is fixed to the gear portion 133 a when thebase side of the engaging portion 133 b is inserted into the insertionhole 133 c of the gear portion 133 a. Also, a pin protruding from thecircumference surface on the base side of the engaging portion 133 b isinserted into the pin groove 133 d of the gear portion 133 a to preventidling of the engaging portion 133 b in the insertion hole 133 c.

In the image forming apparatus having the above-described configuration,a slight rattling movement may be produced between the gear portion 133a and the engaging portion 133 b inserted thereto due to an error in thedimensional accuracy of the gear portion 133 a or the engaging portion133 b. Due to this rattling, the velocity fluctuation pattern of thephotosensitive element gear 133 per rotation slightly varies fromrotation to rotation. As a result, the drive velocity pattern detectedbased on the predetermined detection toner images as described abovedoes not match the actual velocity fluctuation pattern during imageformation, which makes prevention of image displacement difficult.

SUMMARY

This patent specification describes a novel image forming apparatus thatincludes a plurality of image bearing members to bear visible images onrotating surfaces thereof, a plurality of drive sources to individuallydrive the image bearing members, each of the plurality of drive sourcesincluding a drive gear, a plurality of driven gears to individuallyengage the image bearing members on rotation axes of the image bearingmembers and mesh with the drive gears, a visible image forming unit toform the visible images on each of the image bearing members based onimage information, an endless traveling member to endlessly move asurface thereof to sequentially pass positions facing the image bearingmembers, a transfer unit to transfer the visible images formed on eachof the surfaces of the image bearing members to a recording medium heldon the surface of the endless traveling member or to the surface of theendless traveling member and to the recording medium, an image detectionunit to detect the visible images formed on the surface of the endlesstraveling member, and a controller to control rotation of the pluralityof image bearing members according to a velocity fluctuation pattern ofeach surface of the image bearing members based on a detection timeinterval between predetermined visible detection images formed on thesurface of the image bearing member and transferred therefrom to theendless traveling member detected by the image detection unit. Each ofthe plurality of driven gears includes a gear portion and an engagingportion integrated therewith. The gear portion includes a gearedcircumference and the engaging portion engages the image bearing member.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a perspective view illustrating a photosensitive element and aphotosensitive element gear included in a typical image formingapparatus;

FIG. 2 is an exploded perspective view of the photosensitive elementgear shown in FIG. 1;

FIG. 3 is a diagram illustrating a schematic configuration of a printeraccording to a first embodiment of the present invention;

FIG. 4 is an enlarged view illustrating a process unit of the printershown in FIG. 3;

FIG. 5 is a perspective view illustrating the process unit shown in FIG.4;

FIG. 6 is a perspective view illustrating a development unit included inthe process unit;

FIG. 7 is a perspective view illustrating a drive transmission unitfixed in the printer;

FIG. 8 is a plan view illustrating the drive transmission unit;

FIG. 9 is a perspective view illustrating one end of the process unit;

FIG. 10 is a perspective view illustrating a photosensitive element gearand peripheral components included in the printer;

FIG. 11 is a perspective view illustrating the photosensitive elementgear and the peripheral components included in the printer as viewedfrom a process drive motor side;

FIG. 12 is a side view illustrating four photosensitive elements, atransfer unit, and an optical writing unit included in the printer;

FIG. 13 is a diagram illustrating detection toner images for K formed inthe printer;

FIG. 14 is a perspective view illustrating the transfer unit and anoptical sensor unit;

FIG. 15 is a diagram for illustrating a relation between a writingposition of latent images and a transfer position of toner images;

FIG. 16 is a graph illustrating fluctuation in velocity of thephotosensitive element at the writing position;

FIG. 17 is a graph illustrating fluctuation in gap between the latentimages at the writing position;

FIG. 18 is a graph illustrating fluctuation in velocity of thephotosensitive element at the transfer position;

FIG. 19 is a graph illustrating fluctuation in gap between the tonerimages at the transfer position;

FIG. 20 is a graph illustrating relation between fluctuation in velocityof the photosensitive element at the writing position and fluctuation invelocity of the photosensitive element at the transfer position;

FIG. 21 is a graph illustrating relation between fluctuation in gapbetween the latent images at the writing position and fluctuation in gapbetween the toner images at the transfer position;

FIG. 22 is a graph illustrating a variation in the gap between thedetection toner images; and

FIG. 23 is a perspective view illustrating the photosensitive elementgear included in the printer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In describing preferred embodiments illustrated in the drawings,specific terminology is employed for the sake of clarity. However, thedisclosure of this patent specification is not intended to be limited tothe specific terminology so selected and it is to be understood thateach specific element includes all technical equivalents that operate ina similar manner.

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views thereof,particularly to FIG. 23, image forming apparatuses according toexemplary embodiments of the present invention are described.

FIG. 3 is a diagram illustrating a schematic configuration of anelectrophotographic printer (hereinafter referred to as printer)according to an embodiment of the present invention. The printerincludes four process units 1Y, 1C, 1M, and 1K that form toner images ofyellow, cyan, magenta, and black, which are abbreviated as Y, C, M, andK, respectively. The abbreviations may be omitted as necessary. Theprocess units 1Y, 1C, 1M, and 1K have the same configuration usingtoners of different colors: Y toner, C toner, M toner, and K toner,respectively. By way of example, the process unit 1Y is described. Theprocess unit 1Y that forms a Y toner image includes a photosensitiveelement unit 2Y and a development unit 7Y as illustrated in FIG. 4. Thephotosensitive element unit 2Y and the development unit 7Y areintegrated as the process unit 1Y and attached to and detached from theprinter as illustrated in FIG. 5. The development unit 7Y can beattached to and detached from the photosensitive element unit 2Y whenthe process unit 1Y is detached from the printer as illustrated in FIG.6.

Referring to FIG. 4, the photosensitive element unit 2Y includes adrum-like photosensitive element 3Y, a drum cleaning device 4Y, adischarging device, not shown, and a charging device 5Y.

The photosensitive element 3Y is rotated clockwise in FIG. 4 by a driveunit, not shown, and uniformly charged by the charging device 5Y. In thecharging device 5Y, a charging roller 6Y is provided in the vicinity ofthe photosensitive element 3Y. The charging roller 6Y is rotationallydriven counterclockwise in FIG. 4 and a charging bias is applied to thecharging roller 6Y from a power supply, not shown, to uniformly chargethe surface of the photosensitive element 3Y. Instead of the chargingroller 6Y, a charging brush may be used in such a manner that thecharging brush is in contact with the photosensitive element 3Y.Alternatively, the photosensitive element 3Y may be uniformly chargedusing a charger such as a scorotron. The uniformly charged surface ofthe photosensitive element 3Y is then scanned and irradiated with alaser beam L emitted from an optical writing unit 20 to form and bear alatent electrostatic image for Y thereon.

The development unit 7Y includes a first container 9Y and a secondcontainer 14Y. The first container 9Y includes a first conveying screw8Y and the second container 14Y includes a toner density sensor 10Yformed of a magnetic permeability sensor, a second conveying screw 11Y,a development roller 12Y, and a doctor blade 13Y. The first container 9Yand the second container 14Y hold a Y developer, not shown, thatcontains a magnetic carrier and negatively chargeable Y toner. The firstconveying screw 8Y is rotated by a drive unit, not shown, to convey theY developer in the first container 9Y from front to rear in FIG. 4. TheY developer is introduced into the second container 14Y through anopening, not shown, in a partition wall that separates the firstcontainer 9Y from the second container 14Y.

The second conveying screw 11Y in the second container 14Y is rotated bya drive unit, not shown, to convey the Y developer from rear to front inFIG. 4. While conveying the Y developer, the toner density sensor 10Yfixed to the bottom of the second container 14Y detects the tonerdensity in the Y developer. Above the second conveying screw 11Y, thedevelopment roller 12Y is provided in parallel to the second conveyingscrew 11Y. The development roller 12Y includes a development sleeve 15Yserving as a developer member. The development sleeve 15Y is formed of anonmagnetic tube and rotated counterclockwise in FIG. 4. The developmentsleeve 15Y includes a magnet roller 16Y that exerts magnetic force todraw some of the Y developer conveyed by the second conveying screw 11Yto the surface of the development sleeve 15Y. The doctor blade 13Ymaintains a particular gap from the development sleeve 15Y and regulatesthe thickness of the Y developer layer. When the Y developer is conveyedto a development region facing the photosensitive element 3Y, the Ytoner is attracted to the latent electrostatic image for Y on thephotosensitive element 3Y to form a Y toner image thereon. After the Ytoner is thus consumed by the development, the Y developer is returnedto the second conveying screw 11Y as the development sleeve 15Y rotates.At the front end side of the second conveying screw 11Y of FIG. 4, the Ydeveloper is returned to the first container 9Y through an opening, notshown.

The detection result of the magnetic permeability of the Y developerdetected by the toner density sensor 10Y is transmitted as a voltagesignal to a control unit, not shown. This control unit includes a CPU(Central Processing Unit) serving as a computing unit, a RAM (RandomAccess Memory) serving as a data storage unit, and a ROM (Read OnlyMemory) and performs various arithmetic processing and executes controlprograms. Since the magnetic permeability of the Y developer correlateswith the Y toner density of the Y developer, the voltage output from thetoner density sensor 10Y corresponds to the Y toner density. The RAMincluded in the control unit stores data of a target value V_(tref) forY of the output voltage from the toner density sensor 10Y and targetvalues V_(tref) for C, V_(tref) for M, and V_(tref) for K of outputvoltages from the toner density sensors 10C, 10M, and 10K, respectively.As for the development unit 7Y, the value of the output voltage from thetoner density sensor 10Y is compared with the V_(tref) for Y and a tonersupply device for Y, not shown, is driven for a period of time that isdetermined based on the comparison result. Accordingly, Y toner isreplenished in the first container 9Y so that Y toner consumed duringdevelopment is replenished. Therefore, the Y toner density of the Ydeveloper in the second container 14Y is maintained in a particularrange. As for the developers in the process units 1C, 1M, and 1K, tonerreplenishment is controlled in the same manner.

The Y toner image formed on the photosensitive element 3Y, which servesas an image bearing member and a latent electrostatic image bearingmember, is transferred to an intermediate transfer belt 41 (intermediatetransfer process). The drum cleaning device 4Y in the photosensitiveelement unit 2Y removes toner remaining on the surface of thephotosensitive element 3Y after the intermediate transfer process.Thereafter, the discharging device, not shown, discharges the surface ofthe photosensitive element 3Y, such that the surface of thephotosensitive element 3Y is initialized and readied for the next imageformation. C, M, and K toner images are formed on the photosensitiveelements 3C, 3M, and 3K in the process units 1C, 1M, and 1K,respectively, and transferred to the intermediate transfer belt 41 inthe same way.

Below the process units 1Y, 1C, 1M, and 1K, the optical writing unit 20is provided. The optical writing unit 20 serves as a latent imageforming unit and irradiates the photosensitive elements 3Y, 3C, 3M, and3K in the process units 1Y, 1C, 1M, and 1K with laser beams L based onimage information to form latent electrostatic images for Y, C, M, and Kon the photosensitive elements 3Y, 3C, 3M, and 3K, respectively. Theoptical writing unit 20 irradiates the photosensitive elements 3Y, 3C,3M, and 3K by way of a plurality of optical lenses and mirrors includinga polygon mirror 21 that is rotated by a motor and deflects the laserbeams L emitted from an optical source. Alternatively, the irradiationmay be performed by using an LED array.

Below the optical writing unit 20, a first paper feed cassette 31 and asecond paper feed cassette 32 are disposed one above the other in theupright direction. The first paper feed cassette 31 and the second paperfeed cassette 32 store a plurality of recording media P therein, with afirst paper feed roller 31 a and a second paper feed roller 32 a beingin contact with the uppermost recording media P in respective paper feedcassettes. When the first paper feed roller 31 a is rotatedcounterclockwise by a drive unit, not shown, the uppermost recordingmedium P in the first paper feed cassette 31 is output to a paper feedpath 33 extending vertically along the right side of the first paperfeed cassette 31. When the second paper feed roller 32 a is rotatedcounterclockwise by a drive unit, not shown, the uppermost recordingmedium P in the second paper feed cassette 32 is output to the paperfeed path 33. In the paper feed path 33, a plurality of conveyancerollers 34 is provided. The recording medium P fed into the paper feedpath 33 is conveyed upward while being pinched between the conveyancerollers 34.

At the end of the paper feed path 33, a pair of registration rollers 35is provided. The registration rollers 35 pinch the recording medium Pconveyed by the conveyance rollers 34 and immediately suspend theirrotation. The registration rollers 35 convey the recording medium P to asecondary transfer nip, which is described below, at an appropriatetiming.

Above the process units 1Y, 1C, 1M, and 1K, a transfer unit 40 isprovided. In the transfer unit 40, the intermediate transfer belt 41,which is an endless traveling member, is stretched and endlessly movescounterclockwise. The transfer unit 40 also includes a belt cleaningunit 42, a first bracket 43, and a second bracket 44. The transfer unit40 further includes four primary transfer rollers 45Y, 45C, 45M, and45K, a secondary transfer back-up roller 46, a drive roller 47, anauxiliary roller 48, and a tension roller 49. The intermediate transferbelt 41 is stretched over these eight rollers and endlessly movedcounterclockwise by the driven roller 47. Each of the four primarytransfer rollers 45Y, 45C, 45M, and 45K and each of the correspondingphotosensitive elements 3Y, 3C, 3M, and 3K form a primary transfer nipwith the intermediate transfer belt 41 therebetween. A transfer biaswith a reverse polarity to that of the toner, for example, a positivepolarity, is applied to the back side (inner circumference side) of theintermediate transfer belt 41. The Y, C, M, and K toner images on thephotosensitive elements 3Y, 3C, 3M, and 3K are primarily transferred toand superimposed one atop another on the front side of the intermediatetransfer belt 41 while passing through the primary transfer nips for Y,C, M, and K according to the endless movement of the intermediatetransfer belt 41, thereby forming a four-color superimposed toner image(hereinafter referred to as a four-color toner image) on theintermediate transfer belt 41.

The secondary transfer back-up roller 46 and a secondary transfer roller50 provided outside the loop of the intermediate transfer belt 41 formthe secondary transfer nip with the intermediate transfer belt 41therebetween. The registration rollers 35 convey the recording medium Ppinched therebetween to the secondary transfer nip in sync with thefour-color toner image on the intermediate transfer belt 41. Thefour-color toner image on the intermediate transfer belt 41 issecondarily transferred onto the recording medium P all at once by thesecondary transfer electric field and nip pressure generated between thesecondary transfer roller 50 to which a secondary transfer bias isapplied and the secondary transfer back-up roller 46. The four-colortoner image forms a full color image, with the color of the recordingmedium P as background.

After the intermediate transfer belt 41 passes through the secondarytransfer nip, toner that has not been transferred to the recordingmedium P remains attached to the intermediate transfer belt 41. Thisresidual toner is removed by the belt cleaning unit 42. The beltcleaning unit 42 includes a cleaning blade 42 a. The cleaning blade 42 ais in contact with the front side of the intermediate transfer belt 41and scrapes off the toner remaining thereon.

The first bracket 43 in the transfer unit 40 is configured to swing at aparticular degree relative to the rotation axis of the auxiliary roller48 by switching of a solenoid, not shown. When a monochrome image isformed, the first bracket 43 is rotated slightly counterclockwise bydriving of the solenoid so that the primary transfer rollers 45Y, 45C,and 45M are rotated counterclockwise relative to the rotation axis ofthe auxiliary roller 48. Accordingly, the intermediate transfer belt 41is separated from the photosensitive elements 3Y, 3C, and 3M and onlythe process unit 1K is driven to form a monochrome image. Consequently,the process units 1Y, 1C, and 1M are not driven during monochrome imageformation, and thus it is possible to avoid excessive wear on theprocess units 1Y, 1C, and 1M.

Above the secondary transfer nip, a fixing unit 60 is provided. Thefixing unit 60 includes a pressure and heat roller 61 including a heatsource, such as a halogen lamp, and a fixing belt unit 62. The fixingbelt unit 62 includes a fixing belt 64 serving as a fixing member, aheat roller 63 including a heat source such as a halogen lamp, a tensionroller 65, a drive roller 66, and a temperature sensor, not shown. Theendless fixing belt 64 is stretched around the heat roller 63, thetension roller 65, and the drive roller 66, and endlessly movedcounterclockwise. During such endless movement, the back side of thefixing belt 64 is heated by the heat roller 63. The pressure and heatroller 61 rotating clockwise is in contact with the front side of thefixing belt 64 at a position where the fixing belt 64 is suspendedaround the heat roller 63, thereby forming a fixing nip therebetween.

The temperature sensor, not shown, is provided outside the loop of thefixing belt 64 to face the front side of the fixing belt 64 across apresent gap. The temperature sensor detects the surface temperature ofthe fixing belt 64 immediately before the fixing belt 64 enters thefixing nip. The detection result is transmitted to a fixing power supplycircuit, not shown. Based on the detection result, the fixing powersupply circuit controls power supply to the heat sources included in theheat roller 63 and in the pressure and heat roller 61. Therefore, thesurface temperature of the fixing belt 64 is maintained at approximately140 degrees.

In FIG. 3, the recording medium P, once past the secondary transfer nip,is separated from the intermediate transfer belt 41 and forwarded to thefixing unit 60. In the fixing unit 60, while the recording medium P ispinched in the fixing nip and conveyed upward, the recording medium P isheated and pressed by the fixing belt 64 to fix the full color tonerimage thereon.

After the fixing, the recording medium P is discharged from the printervia a pair of discharge rollers 67. A stack portion 68 is formed on theupper surface of the printer and the recording medium P is stackedthereon.

Above the transfer unit 40, four toner cartridges 100Y, 100C, 100M, and100K are provided to hold the Y, C, M, and K toners therein. The Y, C,M, and K toners in the toner cartridges 100Y, 100C, 100M, and 100K aresupplied to the development units 7Y, 7C, 7M, and 7K in the processunits 1Y, 1C, 1M, and 1K, respectively. The toner cartridges 100Y, 100C,100M, and 100K are detachably installed in the printer, separately fromthe process units 1Y, 1C, 1M, and 1K.

FIG. 7 is a perspective view illustrating a drive transmission unitfixed in the printer. FIG. 8 is a plan view illustrating the drivetransmission unit as viewed from above. Four process drive motors 120Y,120C, 120M, and 120K serving as drive sources are fixed to a supportplate that is provided in a standing manner in the printer. To therotation shafts of the process drive motors 120Y, 120C, 120M, and 120K,drive gears 121Y, 121C, 121M, and 121K are coupled, respectively, torotate coaxially with the process drive motors 120Y, 120C, 120M, and120K.

Below the rotation shafts of the process drive motors 120Y, 120C, 120M,and 120K, development gears 122Y, 122C, 122M, and 122K are provided,respectively. The development gears 122Y, 122C, 122M, and 122K mayrotate in engagement with fixed shafts, not shown, provided to thesupport plate in a protruding manner. The development gears 122Y, 122C,122M, and 122K include first gear portions 123Y, 123C, 123M, and 123Kand second gear portions 124Y, 124C, 124M, and 124K, respectively. Eachof the first gear portions 123Y, 123C, 123M, and 123K rotates coaxiallywith each of the second gear portions 124Y, 124C, 124M, and 124K,respectively. The second gear portions 124Y, 124C, 124M, and 124K areprovided on one end of the rotation shafts of the process drive motors120Y, 120C, 120M, and 120K, compared with the first gear portions 123Y,123C, 123M, and 123K. The development gears 122Y, 122C, 122M, and 122Krotate about the fixed shafts by rotation of the process drive motors120Y, 120C, 120M, and 120K by meshing the first gear portions 123Y,123C, 123M, and 123K with the drive gears 121Y, 121C, 121M, and 121K,respectively.

Each of the process drive motors 120Y, 120C, 120M, and 120K is formedof, for example, a DC servo motor, which is one type of DC brushlessmotor, or a stepping motor. The speed reduction ratio between each ofthe drive gears 121Y, 121C, 121M, and 121K and each of photosensitiveelement gears 133Y, 133C, 133M, and 133K is, for example, 1:20. A singlestep reduction between the drive gear and the photosensitive elementgear is to reduce a number of components, costs, and factors causingvariation in transmission due to a mesh error or an eccentricity of thegears. To achieve the relatively large speed reduction ratio of 1:20with the single step reduction, the photosensitive element gear isformed with a larger diameter than the photosensitive element. By usingthe photosensitive element gear with a large diameter, the pitch erroron the surface of the photosensitive element that corresponds to eachtooth meshed of the photosensitive element gear is reduced, andtherefore the effect of uneven print density (banding) in thesub-scanning direction can be reduced. The speed reduction ratio isdetermined based on a speed range that achieves high efficiency and highaccuracy rotation of the photosensitive element according to therelation between a target speed of the photosensitive element and motorcharacteristics.

On the left side of the development gears 122Y, 122C, 122M, and 122K,first relay gears 125Y, 125C, 125M, and 125K are provided. The firstrelay gears 125Y, 125C, 125M, and 125K rotate in engagement with fixedshafts, not shown, by meshing with the second gear portions 124Y, 124C,124M, and 124K and receiving rotary drive forces from the developmentgears 122Y, 122C, 122M, and 122K. With the first relay gears 125Y, 125C,125M, and 125K, the second gear portions 124Y, 124C, 124M, and 124K aremeshed on the upstream side relative to the direction of drivetransmission and clutch input gears 126Y, 126C, 126M, and 126K aremeshed on the downstream side relative to the direction of drivetransmission. The clutch input gears 126Y, 126C, 126M, and 126K aresupported by development clutches 127Y, 127C, 127M, and 127K,respectively, that engage clutch shafts to transmit the rotary driveforces of the clutch input gears 126Y, 126C, 126M, and 126K to theclutch shafts or allow the clutch input gears 126Y, 126C, 126M, and 126Kto idle according to on/off control of power supply by a control unit,not shown. On each end of the clutch shafts of the development clutches127Y, 127C, 127M, and 127K, clutch output gears 128Y, 128C, 128M, and128K are fixed, respectively. When power is supplied to the developmentclutches 127Y, 127C, 127M, and 127K, the rotary drive forces of theclutch input gears 126Y, 126C, 126M, and 126K are transmitted to theclutch shafts so that the clutch output gears 128Y, 128C, 128M, and 128Krotate. When the power supply to the development clutches 127Y, 127C,127M, and 127K is cut, the process drive motors 120Y, 120C, 120M, and120K may be rotating, however, the clutch input gears 126Y, 126C, 126M,and 126K idle on the clutch shafts so that rotation of the clutch outputgears 128Y, 128C, 128M, and 128K comes to a stop.

On the right side of the clutch output gears 128Y, 128C, 128M, and 128K,second relay gears 129Y, 129C, 129M, and 129K are provided,respectively. The second relay gears 129Y, 129C, 129M, and 129K mayrotate in engagement with fixed shafts, not shown, by meshing with theclutch output gears 128Y, 128C, 128M, and 128K.

FIG. 9 is a perspective view illustrating one end of the process unit1Y. The shaft member of the development sleeve 15Y included in thecasing of the development unit 7Y pierces the side of the casing andprotrudes therefrom. To the protruding shaft member, a sleeve upstreamgear 131Y is fixed. A fixed shaft 132Y also protrudes from the side ofthe casing. A third relay gear 130Y is meshed with the sleeve upstreamgear 131Y while rotatably engaging the fixed shaft 132Y.

When the process unit 1Y is installed in the printer, the sleeveupstream gear 131Y and the second relay gear 129Y of FIGS. 7 and 8 aremeshed with the third relay gear 130Y. The rotary drive force of thesecond relay gear 129Y is sequentially transmitted to the third relaygear 130Y and the sleeve upstream gear 131Y, thereby rotating thedevelopment sleeve 15Y.

It should be noted that although only the process unit 1Y is describedwith reference to the drawings, rotary drive forces are transmitted tothe development sleeves in the process units 1C, 1M, and 1K in the sameway as described above with respect to the process unit 1Y.

Although only one end of the process unit 1Y is illustrated in FIG. 9,the other end of the shaft member of the development sleeve 15Y piercesthe other side of the casing and protrudes therefrom. A sleevedownstream gear, not shown, is fixed to the protrusion. The shaftmembers of the first conveying screw 8Y and the second conveying screw11Y also pierce the other side of the casing and protrude therefrom. Afirst screw gear, not shown, and a second gear, not shown, are fixed tothe protrusions, respectively. When the development sleeve 15Y isrotated by drive transmission by the sleeve upstream gear 131Y, thesleeve downstream gear is rotated at the other end side. The secondconveying screw 11Y is rotated by receiving the drive force with thesecond screw gear that meshes with the sleeve downstream gear, and thefirst conveying screw 8Y is rotated by receiving the drive force withthe first screw gear that meshes with the second screw gear. The processunits 1C, 1M, and 1K have the same configuration.

FIG. 10 is a perspective view illustrating the photosensitive elementgear 133Y and peripheral components included in the printer. The firstgear portion 123Y in the development gear 122Y and the photosensitiveelement gear 133Y, which is a driven gear, mesh with the drive gear 121Yfixed to the motor shaft of the process drive motor 120Y. Thephotosensitive element gear 133Y is rotatably supported by the drivetransmission unit and has a larger diameter than that of thephotosensitive element. When the process drive motor 120Y rotates, therotary drive force is transmitted from the drive gear 121Y to thephotosensitive element gear 133Y via a single step reduction to rotatethe photosensitive element. The process units 1C, 1M, and 1K have thesame configuration.

The rotation shaft of the photosensitive element in the process unit andthe photosensitive element gear supported in the printer are connectedby a coupling.

In the printer having the above-described configuration, thephotosensitive element gear 133Y is rotated by the process drive motor120Y and the velocity of the photosensitive element 3Y may fluctuate dueto an eccentricity of the photosensitive element gear 133Y. The velocityvaries in sine wave form with a cycle of a rotation lap of thephotosensitive element 3Y.

Referring to FIG. 3, fluctuation in the linear velocity of each of thephotosensitive elements 3Y, 3C, 3M, and 3K causes fluctuation in thetime taken for the latent image formed on each of the photosensitiveelements 3Y, 3C, 3M, and 3K to move from the position irradiated withlight by the optical writing unit 20 to the primary transfer nip viadevelopment of the latent image. As a result, the images are subtlydisplaced on top of each other at the primary transfer nips.

FIG. 11 is a perspective view illustrating the photosensitive elementgear 133Y and the peripheral components thereof as viewed from theprocess drive motor, and FIG. 12 is a side view illustrating thephotosensitive elements 3Y, 3C, 3M, and 3K, the transfer unit 40, andthe optical writing unit 20. A marking blade member 134Y protrudes at aparticular position in the direction of rotation of the gear portion ofthe photosensitive element gear 133Y. On the side of the photosensitiveelement gear 133Y, a position sensor 135Y is provided. When the rotationof the photosensitive element gear 133Y arrives at a particular angle,the blade member 134Y thereof is located facing the position sensor 135Yand detected by the position sensor 135Y. Therefore, every timing ofwhen the rotation of the photosensitive element gear 133Y arrives at aparticular angle is detected by the position sensor 135Y duringrotation.

Thus, the blade members 134Y, 134C, 134M, and 134K, which are providedto the photosensitive element gears 133Y, 133C, 133M, and 133K rotatingcoaxially with the photosensitive elements 3Y, 3C, 3M, and 3K,respectively, are detected by the position sensors 135Y, 135C, 135M, and135K every time the photosensitive element gears 133Y, 133C, 133M, and133K rotate. The position sensors 135Y, 135C, 135M, and 135K are formedof, for example, photosensors.

Above the transfer unit 40, an optical sensor unit 136 formed of tworeflective photosensors, not shown, arranged at a particular interval inthe width direction of the intermediate transfer belt 41 is providedfacing the upper stretched surface of the intermediate transfer belt 41,with a particular gap therebetween.

A control unit, not shown, of the printer controls detection offluctuation in linear velocity of each photosensitive element caused byan eccentricity of the photosensitive element gear to detect thevelocity fluctuation pattern per rotation lap of the photosensitiveelement gear. The control unit performs such control when an operationthat changes the velocity fluctuation pattern is made, for example, whenthe process unit is replaced, when a print command is issued in a printmode for high quality image, etc.

The control of the fluctuation pattern detection includes formingdetection images on each of the photosensitive elements 3Y, 3C, 3M, and3K and transferring the detection images to the intermediate transferbelt 41, such that the detection images are not superimposed one atopanother. As illustrated in FIG. 13, for example, a detection image PVkfor K includes a plurality of K detection toner images tk01, tk02, tk03,tk04, tk05, tk06, etc. that are transferred to and arranged on theintermediate transfer belt 41 at a particular pitch Ps in the directionof movement of the intermediate transfer belt 41 (sub-scanningdirection) indicated by an arrow in FIG. 13. It should be noted thatalthough the detection toner images are arranged at a particular pitchin theory, in practice, however, the pitch between the K toner imagesvaries according to the fluctuation in the velocity of thephotosensitive element 3K.

Referring to FIG. 3, each detection toner image formed on theintermediate transfer belt 41 passes through a position facing thesecondary transfer roller 50 while being conveyed to a position facingthe optical sensor unit 136 according to the endless movement of theintermediate transfer belt 41. Before the toner image passes through theposition facing the secondary transfer roller 50, the secondary transferroller 50 is separated from the intermediate transfer belt 41 by aroller separation mechanism, not shown, to prevent the detection tonerfrom being transferred to the secondary transfer roller 50.

Each detection toner image is detected by the optical sensor unit 136when passing under the optical sensor unit 136 with the movement of theintermediate transfer belt 41. Therefore, the variation in the detectiontime interval between the detection toner images is detected for eachcolor. The variation in the detection time interval corresponds to thefluctuation in the velocity of the photosensitive element caused by theeccentricity of the photosensitive element gear.

The control unit, not shown, of the printer analyzes the velocityfluctuation pattern per rotation lap of each photosensitive element gearbased on the above-described variation in the detection time intervaland one rotation cycle of the photosensitive element gear. The velocityfluctuation pattern can be analyzed by analyzing amplitude and phase ofthe fluctuation component from a zero cross point or a peak value of thefluctuation by assuming that an average of all data is zero. However,this method is impractical in that the variation increases because thedetected data is greatly affected by noise. Therefore, the printeremploys an orthogonal detection method to analyze the velocityfluctuation pattern. The orthogonal detection allows analysis of thevelocity fluctuation pattern with a small amount of fluctuation data,which is difficult with calculation by detection of a zero cross pointor a peak value of the fluctuation.

In the printer, the phase of the waveform of the velocity fluctuationpattern of the photosensitive element gear is synchronized with that ofthe velocity fluctuation pattern of the photosensitive element with alittle difference in amplitude. Therefore, the velocity fluctuationpattern of the photosensitive element can be detected by detecting thevelocity fluctuation pattern of the photosensitive element gear.

As illustrated in FIG. 14, the detection image PVk for K and thedetection image PVm for M are formed at both ends of the intermediatetransfer belt 41 in the width direction thereof to reduce the time takenfor controlling the fluctuation pattern detection. Specifically, thedetection image PVk formed at one end in the width direction of theintermediate transfer belt 41 is detected by a first optical sensor 137included in the optical sensor unit 136 and the detection image PVmformed at the other end in the width direction of the intermediatetransfer belt 41 is detected by a second optical sensor 138 included inthe optical sensor unit 136. Accordingly, the variation in the detectiontime interval between the K detection toner images in the detectionimage PVk and the variation in the detection time interval between the Mdetection toner images in the detection image PVm are simultaneouslydetected. Therefore, the time taken for controlling the fluctuationpattern detection is reduced. In the same way, the variation in thedetection time interval for Y and the variation in the detection timeinterval for C are simultaneously detected.

After detecting the velocity fluctuation pattern per rotation lap ofeach photosensitive element gear by controlling the fluctuation patterndetection, the control unit of the printer analyzes a drive velocitypattern to cancel the fluctuation in the velocity of the photosensitiveelement gear for each color. The detected velocity fluctuation patternis different from the actual velocity fluctuation pattern of thephotosensitive element gear, which is described below.

Referring to FIG. 15, the latent electrostatic image for Y is formed onthe photosensitive element 3Y by irradiation of light from the opticalwriting unit 20. On the rotational trajectory of the photosensitiveelement 3Y, the position where the latent image is formed by the lightfrom the optical writing unit 20 is indicated by Sa, which is referredto as a writing position Sa. The position where the toner image for Y istransferred to the intermediate transfer belt 41 is indicated by Sb,which is referred to as a transfer position Sb.

It is preferable to form the Y detection toner images at equal intervalsin the circumferential direction of the photosensitive element 3Y.Therefore, the light for forming each Y latent image, which is aprecursor of the Y toner image, is emitted at equal intervals. When thevelocity of the photosensitive element 3Y fluctuates, the gap (distance)between the Y latent images varies according to the fluctuation in thevelocity. Specifically, when the surface of the photosensitive element3Y moves faster than usual at the writing position Sa, the gap betweenthe Y latent images is increased. When the surface of the photosensitiveelement 3Y moves slower than usual, the gap between the Y latent imagesis reduced. Therefore, when the surface velocity of the photosensitiveelement 3Y fluctuates at the writing position Sa as illustrated in FIG.16, the gap between the Y latent images fluctuates as illustrated inFIG. 17. As can be seen in FIGS. 16 and 17, the fluctuation in thevelocity and the fluctuation in the gap between the Y latent images arein phase with each other.

When the velocity of the photosensitive element 3Y fluctuates duringprimary transfer of the Y toner images obtained by developing the Ylatent images to the intermediate transfer belt 41, the Y toner images,which may be formed on the photosensitive element 3Y at equal intervals,are transferred to the intermediate transfer belt 41 at unequalintervals. When the surface of the photosensitive element 3Y movesfaster than usual at the transfer position Sb, the gap between the Ytoner images on the intermediate transfer belt 41 is reduced. When thesurface of the photosensitive element 3Y moves slower than usual, thegap between the Y toner images on the intermediate transfer belt 41 isincreased. Therefore, when the surface velocity of the photosensitiveelement 3Y fluctuates at the transfer position Sb as illustrated in FIG.18, the gap between the Y toner images on the intermediate transfer belt41 fluctuates as illustrated in FIG. 19. As can be seen in FIGS. 18 and19, the fluctuation in the velocity and the fluctuation in the gapbetween the Y toner images are 180 degrees out of phase with each other.

As a result, fluctuation caused by the fluctuation in the surfacevelocity of the photosensitive element at the writing position Sa andfluctuation caused by the fluctuation in the surface velocity of thephotosensitive element at the transfer position Sb are overlapped toproduce the variation in the gap between the Y detection toner images onthe intermediate transfer belt 41. Specifically, referring to FIG. 15,an angle between the writing position Sa and the transfer position Sb tothe center of the photosensitive element 3Y is assumed to be α°. Then,as illustrated in FIG. 20, the phase of the fluctuation in the surfacevelocity of the photosensitive element at the writing position Saindicated by the dashed line and the phase of the fluctuation in thesurface velocity of the photosensitive element at the transfer positionSb indicated by the continuous line are α° out of phase. Thevelocity-gap relation is reversed when the toner image is transferred.Therefore, as illustrated in FIG. 21, the fluctuation in the gap betweenthe Y latent images indicated by the dashed line and the fluctuation inthe gap between the Y toner images on the intermediate transfer belt 41indicated by the continuous line are 180+α° out of phase.

Therefore, the fluctuation in the gap between the latent images at thewriting position Sa and the fluctuation in the gap between the tonerimages at the transfer position Sb are overlapped to produce thevariation in the gap between the toner images. Therefore, the variationin the gap between the toner images, which is detected based on thevariation in the detection time interval of each detection toner image,has a composite waveform of the two characteristic waveforms indicatedby a dashed dotted line in FIG. 22. The fluctuation in the gap betweenthe latent images at the writing position Sa, i.e. the actual velocityfluctuation pattern of the photosensitive element gear, can be analysedbased on the phase and amplitude of the composite waveform and the180+α° out of phase relation by a known analysis method. By driving theprocess drive motor according to the drive velocity pattern that is inthe opposite phase to the fluctuation in the gap between the latentimages, the fluctuation in the velocity of the photosensitive elementcan be reduced.

Prior to the analysis of the drive velocity pattern, the angle ofrotation of the photosensitive element gear at the start of the waveformof the fluctuation in the gap between the latent images, which is apoint of starting to form a latent image corresponding to a leading endof the detection toner images, is identified. The control unit of theprinter determines a timing of starting forming the latent images forthe detection toner images of each color based on the time (gear angledtime) at which the blade member of the corresponding photosensitiveelement gear is detected by the position sensor.

The determination of the timing of starting forming the latent images isnow described. The control unit starts forming the latent images for thedetection toner images of each color after a period of time t1 from thegear angled time, which is referred to as latent image formation starttime. In other words, the latent image formation start time is the gearangled time plus the period of time t1. Therefore, the waveform of thefluctuation in the gap between the latent images at the writing positionSa (the actual velocity fluctuation pattern of the photosensitiveelement gear) starts at the time after the period of time t1 from whenthe position sensor detects the blade member of the photosensitiveelement gear. By using the start of the waveform as a reference to drivethe process drive motor according to the drive velocity pattern, whichis in the opposite phase to the waveform of the fluctuation in the gapbetween the latent images at the writing position Sa, the fluctuation inthe velocity of the photosensitive element caused by the eccentricitycan be reduced by the fluctuation in the driving velocity.

When the printer forms an image using image information sent from, forexample, a personal computer, the process drive motor for each color iscontrolled based on a pre-analyzed drive velocity pattern and the gearangled time transmitted from the position sensor.

However, this technique may not sufficiently reduce the fluctuation inthe velocity of the photosensitive element for each color in a typicalimage forming apparatus, for the reason described with respect to FIG. 1and FIG. 2 described above.

More specifically, as described above the slight rattling movementproduced between the gear portion 133 a and the engaging portion 133 binserted thereto due to an error in the dimensional accuracy of the gearportion 133 a or the engaging portion 133 b causes the velocityfluctuation pattern of the photosensitive element gear 133 per rotationto vary slightly from rotation to rotation. As a result, the drivevelocity pattern detected based on the predetermined detection tonerimages does not match the velocity fluctuation pattern during imageformation, which causes a mismatch between the drive velocity patternthat is analyzed in the above-described manner and the actual velocityfluctuation pattern of the photosensitive element gear 133 and preventsreduction of fluctuation in the velocity of the photosensitive elementgear 133.

FIG. 23 is a perspective view illustrating the photosensitive elementgear 133Y included in the printer of the present invention. Thephotosensitive element gear 133Y includes a disk-like gear portion 133aY and a tubular engaging portion 133 bY that are formed of the samematerial, for example, a resin material, and seamlessly integrated witheach other. In each of the photosensitive element gear 133C, 133M, or133K, the gear portion and the engaging portion are similarly integratedwith each other to form a single unit.

In the printer, each of the photosensitive element gear 133Y, 133C,133M, and 133K includes the disk-like gear portion having a gearedcircumference and the engaging portion engaging the photosensitiveelement. The gear portion and the engaging portion are integrated witheach other to form a single unit, and thus there is no rattlingtherebetween. As a result, any mismatch between the velocity fluctuationpattern detected based on the detection toner images and the actualvelocity fluctuation pattern during image formation is prevented, andtherefore degradation of superimposition accuracy caused by suchrattling is prevented.

The engaging portion 133 bY of the photosensitive element gear 133Yillustrated in FIG. 23 is a female component that has a concave portionfor receiving the coupling of the photosensitive element. The concaveportion of the female engaging portion 133 bY can be in the form of, forexample, a polygonal prism such as a triangular prism or a quadrangularprism that fits the outside diameter of the coupling, not shown.However, the concave portion in the form of a polygonal prism has arelatively small number of mesh points with the coupling, and a mesherror tends to cause fluctuation in transmission velocity. Therefore, inthe printer, the female engaging portion 133 bY has an inner gearedcircumference in its cylindrical concave portion and the coupling is amale component with gears meshed with the inner gears of the engagingportion 133 bY. With this configuration, fluctuation in transmissionvelocity caused by a mesh error can be reduced by increasing the meshpoints as compared with an engaging portion with the concave portion inthe form of polygonal prism. Alternatively, a female coupling and a maleengaging portion 133 bY can be used, that is, the engaging portion 133bY is a male component and the coupling is a female component with aconcave portion that receives the engaging portion 133 bY.

In the printer in which the photosensitive element gear includes thegear portion and the engaging portion that are integrated with eachother, the velocity fluctuation pattern of the photosensitive element ischanged only when an angle of engagement in the direction of rotationbetween the rotation shaft of the photosensitive element and theengaging portion of the photosensitive element gear is changed byattachment, detachment, or replacement of the process unit. Therefore,it is preferable to configure the control unit to control thefluctuation pattern detection only when the attachment or detachment ofthe process unit is detected, thereby enabling unnecessary downtime tobe eliminated by avoiding unnecessary control of the fluctuation patterndetection.

Although the description given above is of the velocity fluctuationpattern per rotation lap of the photosensitive element, the velocity ofthe photosensitive element may fluctuate periodically in a cycle longerthan one rotation lap, which is a cycle of 2Π/ω×(α×360) seconds, where ωis an angular velocity of the photosensitive element. By providing eachdetection toner image such that the detection toner image is detectedfor a period of time longer than the cycle of the fluctuation, theperiodical fluctuation in the velocity with a cycle longer than onecycle can be detected, and therefore the velocity can be controlledbased on a pattern longer than one cycle.

Next, examples of printers having additional characteristics are nowdescribed. The printers described in the following examples have thesame configuration as that of the above-described embodiment, unlessotherwise specified.

In the printer according to a first example of the present invention, asthe latent image formation start time of the detection toner image foreach color, the control unit uses the corresponding gear angled time,which is the time when the position sensor detects the blade member ofthe photosensitive element gear. In other words, the above-describedperiod of time t1 is zero μsec, thereby enabling the operation of addingthe period of time t1 to the gear angled time to identify the start timeof the drive velocity pattern to be omitted.

In the printer according to a second example of the present invention,when an image is formed using image information, the control unitdetermines, for each process drive motor of each color, whether or notto adjust the driving velocity of the process drive motor according tothe pre-analyzed drive velocity pattern based on a maximum fluctuationin the velocity fluctuation pattern of the photosensitive element gear.Specifically, when the maximum fluctuation in the velocity fluctuationpattern of the photosensitive element gear is at or below a thresholdvalue, the driving velocity of the process drive motor is not adjustedbut the process drive motor is driven at a constant velocity to form animage using image information. When the maximum fluctuation in thevelocity is at or above the threshold value, the driving velocity of theprocess drive motor is adjusted according to the drive velocity pattern.

The reason for performing such control is now described. There is adifference between the time when the blade member of the photosensitiveelement gear is detected by the position sensor (gear angled time) andthe time when the photosensitive element gear is actually rotated at theparticular angle, because there are limits to detection accuracy of theposition sensor and variations in rotation of the process drive motor.Accordingly, a slight fluctuation remains in the linear velocity of thephotosensitive element even after the driving velocity of the processdrive motor is adjusted according to the drive velocity pattern. Whenthere is little fluctuation in the actual velocity of the photosensitiveelement, the adjustment of the driving velocity according to the drivevelocity pattern may actually increase the fluctuation in the velocityof the photosensitive element due to the above-described causes comparedto a case in which the fluctuation in the velocity of the photosensitiveelement is not adjusted. Therefore, when the maximum fluctuation in thevelocity fluctuation pattern is at or below the threshold value, thedriving velocity of the process drive motor is not adjusted but theprocess drive motor is driven at a constant velocity, thereby avoidingany increase in the fluctuation in the velocity of the photosensitiveelement generated by such adjustment.

In the printer according to a third example of the present invention,when detecting the velocity fluctuation pattern for each photosensitiveelement gear of each color, the control unit calculates a period of timefrom when the position sensor detects the blade member of thephotosensitive element gear to when the position sensor detects the nextblade member. The calculation result is stored in the RAM as a referencetime taken for the rotation lap of the photosensitive element gear.

When an image is formed using image information, the control unitcalculates the time taken for the rotation lap of each photosensitiveelement gear of each color and an average of the calculated time. Whenthe difference between the average and the reference time previouslystored in the RAM is at or above a threshold value, the pre-analyzeddrive velocity pattern is corrected. Further, after updating thereference time to the same value as the average, the driving velocity ofthe process drive motor is adjusted according to the corrected drivevelocity pattern.

The reason for performing such control is now described. As describedabove, the drive velocity pattern is analyzed based on the detectionresult of the velocity fluctuation pattern only when the velocityfluctuation pattern of the photosensitive element is changed due to, forexample, replacement of the process unit. Therefore, the time intervalfor updating the drive velocity pattern is relatively long. During sucha time interval, the time taken for the rotation lap of thephotosensitive element gear may be changed due to, for example,degradation of the process drive motor or brief fluctuations in theoutput voltage from a motor power supply. When the driving velocity ofthe process drive motor is adjusted based on the drive velocity patterncorresponding to the unadjusted time taken for the rotation lap of thephotosensitive element gear, the fluctuation in the velocity of thephotosensitive element is not sufficiently reduced. Therefore, when thedifference between the average time taken for the rotation lap of thephotosensitive element gear and the reference time taken for therotation lap is at or above the threshold value, the drive velocitypattern is corrected according to the average. Specifically, theuncorrected waveform of the drive velocity pattern is expanded or shrunkin the time axis direction to have one cycle of the average time.Consequently, increase in the amount of fluctuation in the velocity ofthe photosensitive element, which is caused by an inappropriate drivevelocity pattern due to fluctuation in the time taken for the rotationlap of the photosensitive element gear, is reduced.

Although the description given above is of the printer that adjusts thedriving velocity of the process drive motor according to the drivevelocity pattern, the photosensitive element gear that includes the gearportion and the engaging portion that are integrated with each other toform a single unit can also be applied to an image forming apparatusthat adjusts the phase of the velocity fluctuation pattern of eachphotosensitive element.

In each printer according to the embodiment and each of the examples,the control unit serving as a controller is configured to detect thevelocity fluctuation pattern per rotation lap of the surface of thephotosensitive element serving as an image bearing member. Therefore,the velocity fluctuation pattern per rotation lap of the photosensitiveelement fluctuating due to an eccentricity of the photosensitive elementgear, which is a driven gear, is detected.

In each printer according to the embodiment and each of the examples,each of the position sensors 135Y, 135C, 135M, and 135K serving as arotation angle detection unit is provided to detect when rotation ofeach of the plurality of photosensitive element gears 133Y, 133C, 133M,and 133K arrives at a particular angle, respectively. The control unitis configured to perform control to determine a timing of startingforming each of the detection toner images for Y, C, M, and K on thephotosensitive elements 3Y, 3C, 3M, and 3K, respectively, based on thedetection results of the position sensors 135Y, 135C, 135M, and 135K.Therefore, the start time of the velocity fluctuation pattern and thedrive velocity pattern are identified based on the gear angled timedetected by each of the position sensors 135Y, 135C, 135M, and 135K, andtherefore, the driving velocity of each of the process drive motors120Y, 120C, 120M, and 120K is sufficiently adjusted.

In the printer according to the first example, the time when thephotosensitive element gear is rotated at the particular angle (the gearangled time) is adopted as the latent image formation start time.Therefore, the operation of adding the period of time t1 to the gearangled time to identify the start time of the drive velocity pattern canbe omitted.

As described above, the control unit is configured to perform controlfor each of the plurality of the photosensitive elements 3Y, 3C, 3M, and3K to analyze the drive velocity pattern of each of the process drivemotors 120Y, 120C, 120M, and 120K that cancels the fluctuation in thesurface velocity of the photosensitive element based on the latent imageformation start time and the velocity fluctuation pattern, and adjustthe driving velocity of each of the process drive motors 120Y, 120C,120M, and 120K based on the results of that analysis. Therefore,fluctuation in the surface velocity of the photosensitive element causedby the eccentricity of the photosensitive element gear is cancelled bythe fluctuation in the driving velocity of the process drive motor,thereby reducing fluctuation in the surface velocity of thephotosensitive element and resultant image displacement.

In the printer according to the second example, the control unit isconfigured to perform control for each of the plurality of process drivemotors 120Y, 120C, 120M, and 120K not by adjusting the driving velocityof the process drive motor according to the drive velocity pattern butby driving the process drive motor at a constant velocity when themaximum fluctuation in the velocity fluctuation pattern corresponding tothe process drive motor is at or below a threshold value. Therefore, anyincrease in fluctuation in the velocity of the photosensitive element,which is caused by adjustment of the driving velocity of the processdrive motor, is prevented.

In the printer according to the third example, the control unit isconfigured to perform control for each of the plurality ofphotosensitive element gears 133Y, 133C, 133M, and 133K to calculate atime taken for the rotation lap of the photosensitive element gear at aparticular timing based on the detection result of each of the positionsensors 135Y, 135C, 135M, and 135K and correct the drive velocitypattern according to the calculation result. Therefore, any increase inthe amount of fluctuation in the velocity of the photosensitive element,which is caused by an inappropriate drive velocity pattern due tofluctuation in the time taken for the rotation lap of the photosensitiveelement gear, is reduced.

As can be understood by those skilled in the art, numerous additionalmodifications and variations are possible in light of the aboveteachings. It is therefore to be understood that, within the scope ofthe appended claims, the disclosure of this patent specification may bepracticed otherwise than as specifically described herein.

Further, elements and/or features of different example embodiments maybe combined with each other and/or substituted for each other within thescope of this disclosure and appended claims.

Still further, any one of the above-described and other example featuresof the present invention may be embodied in the form of an apparatus,method, system, computer program or computer program product. Forexample, the aforementioned methods may be embodied in the form of asystem or device, including, but not limited to, any of the structuresfor performing the methodology illustrated in the drawings.

Any of the aforementioned methods may be embodied in the form of aprogram. The program may be stored on a computer-readable medium andadapted to perform any one of the aforementioned methods when run on acomputer device (a device including a processor). The program mayinclude computer-executable instructions for carrying out one or more ofthe steps above, and/or one or more of the aspects of the invention.Thus, the storage medium or computer-readable medium, is adapted tostore information and is adapted to interact with a data processingfacility or computer device to perform the method of any of the abovementioned embodiments.

The storage medium may be a built-in medium installed inside a computerdevice main body or a removable medium arranged so that it can beseparated from the computer device main body. Examples of the built-inmedium include, but are not limited to, rewriteable non-volatilememories, such as ROMs and flash memories, and hard disks. Examples ofthe removable medium include, but are not limited to, optical storagemedia such as CD-ROMs and DVDs; magneto-optical storage media, such asMOs; magnetic storage media, including but not limited to floppy disks(trademark), cassette tapes, and removable hard disks; media with abuilt-in rewriteable non-volatile memory, including but not limited tomemory cards; and media with a built-in ROM, including but not limitedto ROM cassettes, etc. Furthermore, various information regarding storedimages, for example, property information, may be stored in any otherform, or provided in other ways.

Example embodiments being thus described, it will be apparent that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. An image forming apparatus comprising: a plurality of image bearingmembers configured to bear visible images on rotating surfaces thereof;a plurality of drive sources configured to individually drive the imagebearing members, each of the plurality of drive sources comprising adrive gear; a plurality of driven gears configured to individuallyengage the image bearing members on rotation axes of the image bearingmembers and mesh with the drive gears; a visible image forming unitconfigured to form the visible images on each of the image bearingmembers based on image information; an endless traveling memberconfigured to endlessly move a surface thereof to sequentially passpositions facing the image bearing members; a transfer unit configuredto transfer the visible images formed on each of the surfaces of theimage bearing members to a recording medium held on the surface of theendless traveling member, or to the surface of the endless travelingmember and to the recording medium; an image detection unit configuredto detect the visible images formed on the surface of the endlesstraveling member; and a controller configured to control rotation of theplurality of image bearing members according to a velocity fluctuationpattern of each surface of the image bearing members based on adetection time interval between predetermined visible detection imagesformed on the surface of the image bearing member and transferredtherefrom to the endless traveling member detected by the imagedetection unit, wherein each of the plurality of driven gears comprisesa gear portion and an engaging portion integrated therewith, the gearportion having a geared circumference and the engaging portion engagingthe image bearing member.
 2. The image forming apparatus according toclaim 1, wherein the velocity fluctuation pattern is a velocityfluctuation pattern detected based on a rotation lap of the imagebearing member.
 3. The image forming apparatus according to claim 2,wherein the controller determines a timing of starting forming thepredetermined visible detection images on each surface of the imagebearing members based on detection results from each of a plurality ofrotation angle detection units, each such rotation angle detection unitconfigured to detect when rotation of the corresponding driven geararrives at a predetermined angle.
 4. The image forming apparatusaccording to claim 3, wherein the controller starts forming thepredetermined visible detection images when the rotation of the drivengear arrives at the predetermined angle.
 5. The image forming apparatusaccording to claim 3, wherein the controller adjusts a driving velocityof the drive source to a drive velocity pattern that cancels thefluctuation in the surface velocity of the image bearing member obtainedfrom analysis of the timing of starting forming the predeterminedvisible detection images and the velocity fluctuation pattern of thesurface of the image bearing member.
 6. The image forming apparatusaccording to claim 5, wherein the controller drives the drive source ata constant velocity when the maximum fluctuation in the velocityfluctuation pattern corresponding to the drive source is at or below athreshold value.
 7. The image forming apparatus according to claim 5,wherein the controller corrects the drive velocity pattern according toa calculation result obtained by calculating a time taken for therotation lap of the driven gear at a particular timing based on thedetection results of each of the rotation angle detection units for eachof the driven gears.